This invention relates to a riser reactor system, a catalyst return conduit apparatus and a process for reacting a feedstock within the riser reactor system, especially an oxygenate feedstock to produce olefins.
Processes for the preparation of olefins from oxygenates are known in the art. Of particular interest is often the production of light olefins, in particular ethylene and/or propylene. The oxygenate feedstock can for example comprise methanol and/or dimethylether, and an interesting route includes their production from synthesis gas derived from e.g. natural gas or via coal gasification.
For example, WO2007/135052 discloses a process wherein an alcohol and/or ether containing oxygenate feedstock and an olefinic co-feed are reacted in the presence of a zeolite having one-dimensional 10-membered ring channels to prepare an olefinic reaction mixture, and wherein part of the obtained olefinic reaction mixture is recycled as olefinic co-feed. With a methanol and/or dimethylether containing feedstock, and an olefinic co-feed comprising C4 and/or C5 olefins, an olefinic product rich in light olefins can be obtained.
A suitable reactor system for oxygenate-to-olefins reactions includes a riser reactor. Using a riser reactor, a continuous process can be employed where the used catalyst is separated from the product and other fluids in a separation zone, at least some of the catalyst is regenerated to remove some of the coke deposits, and catalyst is reintroduced into the riser reactor via a catalyst return conduit, also referred to as a standpipe. It is important for smooth continuous operation of a riser reactor system that the solids circulation operates well.
US2003/0234209 teaches a method for controlling solids circulation in a gas/solids reaction system. The method entails aerating solid particles in a standpipe, wherein aeration fluid is injected into the standpipe away from the internal wall.
In the design of riser reactor systems, e.g. for oxygenate-to-olefins conversions, it can be desirable to use tall reactors, in order to provide the right reactions conditions in terms of catalyst concentration, superficial velocity, flow regime and/or residence time for a given cross-sectional area of the riser. For commercially interesting throughputs and a desired height aspect ratio between height and cross-sectional area, reactors can be desired to reach heights of 30 m, 50 m, 70 m or even more.
US2004/0076554 addresses a particular problem encountered when designing a reactor system so tall, namely that heavy equipment at the top of such a structure requires expensive support structures. US2004/0076554 discloses a multiple riser reactor, wherein the effluent from risers is laterally fed into a common separation vessel, which is arranged between the riser reactors and not on above them. From the lower end of the separation vessel catalyst guided via a central downcomer to a catalyst retention zone, and from the catalyst retention zone short standpipes feed the catalyst back to the inlets of the multiple risers. In this way the overall height of the structure is reduced.
Applicant has realized another problem in designing tall riser reactors, in particular when a design is used in which a catalyst retention zone, wherein catalyst is collected after separation and perhaps regeneration, is arranged at a very high position, e.g. at the top of a tall riser reactor. In this case the standpipe for feeding catalyst from the catalyst retention zone to the lower end of the riser becomes very tall as well. Applicant has realized that it is problematic to reliably control the flow of fluid in such a standpipe of more than 20 meters height, since valves that are typically used, e.g. slide valves, cannot be used at higher pressure differences than about 1 bar. Otherwise wear would be too high and control unreliable.
There is a need for an improved catalyst return in riser reactors.